Use instant popup for shader execute forwards/backwards buttons

There is no action associated with clicking the button itself, only for
the menu associated with it. Using QToolButton::MenuButtonPopup means
that the majority of the button is for the button's own action, and
there is a small arrow next to the button that shows the menu. Since
there's no action, this means that the small menu arrow needs to be
clicked each time instead of the big button, which is annoying.
QToolButton::InstantPopup instead always shows the menu (though it
doesn't have the arrow indicating a menu exists, although this is not
particularly important).

InstantPopup is also already used for the "Insert" button used when
editing a custom shader for the texture viewer. MenuButtonPopup is used
in many other places throughout RenderDoc, but they are all save or
export buttons where there is a meaningful default action.

qrenderdoc/Windows/ShaderViewer.ui

@@ -439,7 +439,7 @@
         <normaloff>:/control_reverse_blue@2x.png</normaloff>:/control_reverse_blue@2x.png</iconset>
       </property>
       <property name="popupMode">
-       <enum>QToolButton::MenuButtonPopup</enum>
+       <enum>QToolButton::InstantPopup</enum>
       </property>
       <property name="toolButtonStyle">
        <enum>Qt::ToolButtonTextBesideIcon</enum>
@@ -462,7 +462,7 @@
         <normaloff>:/control_play_blue.png</normaloff>:/control_play_blue.png</iconset>
       </property>
       <property name="popupMode">
-       <enum>QToolButton::MenuButtonPopup</enum>
+       <enum>QToolButton::InstantPopup</enum>
       </property>
       <property name="toolButtonStyle">
        <enum>Qt::ToolButtonTextBesideIcon</enum>